Psychological and social processes are of focal importance in the development of sexual behavior that constitutes a risk of HIV infection or transmission, as well as in behavioral risk reduction and its maintenance. A number of theories and models have been formulated to guide the investigator in this area. Given the demographic heterogeneity of populations under study, however, much qualitative/ethnographic work is needed for the selection, adaptation and de-novo-development of assessment instruments that are appropriate, gender-specific, and culturally sensitive. The Psychosocial/Qualitative Core has been designed to bring the expertise of established social and anthropological investigators to bear on this entire research process. The Core will guide and advise all project investigators and assure that psychological and social theory and methods are kept sharply in focus in the design, implementation and evaluation of research assessment.